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Originally Posted by machenotizia
there is a way to compare with a matematical score of difference from encoded video with the original one?
i don't want to compare it subjectively.
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This is huge and religious topic, but big picture is that the mathematical scores that can be calculated don't highly correlate with subjective video quality. The VPx series has historically been highly tuned for the PNSR metric, which is one of the easiest to calculate, but which has one of the lowest correlations to subjective quality ratings. SSIM is another common one which is better, but still not great.
All the standard metrics that get cited are also single-frame only, which means they don't account for how images change between frames. Since that's a critical component of moving video, codecs heavily optimized for SSIM and PSNR will tend to introduce temporal artifacts.
x264 and x265 are primarily optimized towards subjective quality. There is the internal metric of "rate factor," but it's not designed to provide objective comparisons between difference pieces of content or different settings, and isn't available in any other codecs.
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but i don't want to go off topic about vp9 stuttering problems on mobile, if possible.
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Without a hardware decoder, the primary limitation of a software decoder is pixel fill rate (height * width * fps), with some factor from bitrate. The design of VP9 forces a highly serialized decoder design, and lacks b-frame to drop. This means if a single frame is dropped in decode, the rest of the GOP is unplayable.
I'd expect that the limitation of VP9 would be based on single-core performance. Reducing frame size is the easiest way to improve performance, followed by reducing bitrate, and lastly reducing frame rate. Since frame rate needs to be divided by an integer, it's a particularly painful way to reduce CPU load, and is very rarely worth it unless you're already below 320x240.
There have been improvements in VP9 decoder performance, so making sure that a recent one is being used would also help. Although enforcing that for customers would be challenging.